Conventionally, in an electrophotographic image forming apparatus and an electrostatic recording apparatus, an electrical or magnetical latent image is visualized with a toner. For example, in electrophotography, a latent electrostatic image (latent image) is formed on a photoconductor, followed by developing the latent image with the toner, to form a toner image. The toner image is typically transferred onto a transfer medium such as paper, followed by fixing onto the transfer medium by heating or the like.
Recently, there are increasing demands from the market for image forming apparatuses of high speed and energy saving, and therefore a toner having excellent low temperature fixability and capable of providing high quality images is desired.
As a method for achieving the low temperature fixability of the toner, there is a method of lowering the softening point of the binder resin contained in the toner. However, according to this method, a so-called offset (also referred to as hot offset hereinafter) occurs, in which part of a toner image is deposited onto a surface of a fixing member during fixing, and then transferred to photocopy paper. In addition to this, the heat resistant storage stability of the toner degrades, and therefore toner particles are fused to each other particularly in high temperature environments, which is so called blocking.
As for the technique for solving the aforementioned problems, it has been known that a crystalline resin is used as a binder resin of the toner. The crystalline resin is capable of decreasing the softening point of the toner to around the melting point thereof by sharply softening at the melting point of the resin, while maintaining the heat resistant storage stability at the temperature equal to or lower than the melting point. Accordingly, use of the crystalline resin in the toner realizes both the low temperature fixability and heat resistant storage stability at high levels.
As for the toner using the crystalline resin, for example, there is proposed a toner in which a crystalline resin obtained by elongating crystalline polyester with diisocyanate is used as a binder resin (see PTL 1 and PTL 2).
There is proposed a toner using a crystalline resin which has a crosslink structure due to an unsaturated bond containing sulfonic acid groups (see PTL 3).
Moreover, there is proposed a technique related to crystalline resin particles having excellent low temperature fixability and heat resistant storage stability, in which a ratio of a softening point to a peak temperature of heat of melting, and viscoelastic properties are specified (see PTL 4).
There is also proposed a toner containing as the binder resin, crystalline polyester, of which endothermic peak, endothermic amount, and half width of the endothermic peak are prescribed (see PTL 5 and PTL 6).
However, because toners using these conventional crystalline resins rapidly soften at the melting point of the resin, toner particles aggregate with each other in the developing device when heat generated when they are stirred in the developing device increases, causing a problem that the generated coarse toner particles cannot be transferred to the transfer medium to generate voids in the image (white voids).
Therefore, currently, there is a demand for a toner that has excellent low temperature fixability and heat resistant storage stability, and can suppress occurrence of aggregation of toner particles in the developing device.